Not Applicable
1. Field of the Invention
The present invention relates to a golf club shaft having different modal frequencies to improve both the swing feedback and post-impact harshness of a golf club. More specifically, the present invention relates to the improvement of a golf shaft by utilization of one or more layers of foil in specifically oriented directions to increase the performance of the golf club shaft upon impact with a golf ball.
2. Description of the Related Art
Golf clubs are an assembly of a club head, shaft, grip and miscellaneous adapter and/or finish components. The shafts have been made from wood and then metal (steel, aluminum, titanium and metal matrix materials). Composite materials, such as glass/epoxy and carbon/epoxy, have also been utilized. The majority of shafts are now either steel or carbon/epoxy, although hybrid shafts that combine steel or titanium with carbon/epoxy can also be found.
Shafts are designed with various bending and torsional stiffnesses and weights to accommodate customer preferences. Shafts are categorized and marketed by these parameters and the associated club parameter, first frequency. Golf literature attributes a wide range of performance differences to small changes in shaft and shaft driven club parameters, the significant of these parameters being primarily club mass, mass distribution and feel. The shaft role in feel is first in feedback of the inertial forces (resistance) and therefore the path of the head during back swing and down swing. Mass, mass distribution and the first bending mode are of interest for these motions. Secondly, the shaft feel contribution is independent of swing after ball impact. The impact location and energy determines the amplitude of excitation of the various natural modes of the club.
The shaft plays a principal role in defining the mode shapes and frequencies and in transmitting the vibrations to the golfer""s hands. The first mode frequencies have been shown in a range of non-golf studies to be frequencies that reinforce learning and are generally relaxing and pleasurable. These modes are energized for club head-ball impact that acts through or close to the head center of mass. The third mode for most clubs combines bending and torsion. This mode""s natural frequency is typically in a frequency range of 35 to 60 hertz and higher ranges. This particular frequency range matches well with nerve receptors in the hands and is often interpreted by golfers as harsh and unpleasant. The inertial properties of the head affect this club mode, and a high inertia about the shaft axis mass reduces the impact energy driving this mode. For heads with odd inertial coupling or low inertia, the impact will excite the golf clubs, causing a harsh feel, particularly for off-center hits. Modifying the torsional stiffness of a shaft can change the higher frequencies of a golf club and result in an overall improvement in satisfaction. Dampening can increase the decay of a harsh vibration, however, it can also mask the sought after reinforcing feedback. Steel shafts have a high torsional stiffness and are preferred by some players, but lack the low mass and natural dampening of carbon/epoxy shafts. Increasing the torsional stiffness of a shaft can decrease the amplitude of the combined modes and shift frequencies, as it will take more impact energy to achieve the same harshness thresholds. Carbon shafts provide the capability, through fiber selections and combinations along with fiber placement and orientation, to tune the club modes to achieve a generally superior combination of club modes. However, the carbon/epoxy shaft must typically utilize large tube selections, high modulus fibers and high percentages of 45xc2x0 plies to achieve the feel combinations sought by golfers. The diameters have traditionally been the same for steel and carbon shafts, but this is now changing. The cost of higher modulus fibers adds to the production cost of the club. Attempts to improve club feel by increasing passive dampening have had only limited success. In golf clubs, the elastic, loss, and mass properties of the shaft combined with the head, grip and any other components result in structures that have specific vibration mode shapes, frequencies and decays. Some of these frequencies and mode shapes enhance the feel and perception for the golfer. These are typically the lower frequency modes, usually the first and second bending modes.
Mode frequencies are routinely measured in golf clubs and are used as measures of shaft and club quality and performance. Clubs and shafts are fit to specific player segments based on designed to and measured parameters. The parameters include: club frequency in a clamped condition; shaft frequency with a representative head mass; shaft-bending deflection under an arbitrary load case; and shaft deflected profile under an arbitrary load case. These parameters correlate to club modes. The actual frequencies in play are actually different from the static measurements due to an extension force on the shaft pulling the head into a near circular path during a swing.
There remains a need for golf club shafts that have a high torsional stiffness and a low bending stiffness while simultaneously maintaining the frequencies of the club and shaft in a range that is desirable to the golfer.
The present invention provides a golf club shaft that includes a carbon/epoxy shaft body wrapped with one or more layers of foil to increase the torsional stiffness of the shaft while maintaining the golf club""s modal frequencies in a range that is more desirable to the golfer than other golf clubs.
One aspect of the present invention is the use of a nearly standard carbon/epoxy shaft with one or more layers of steel, steel alloy, titanium, titanium alloy or other metal foils. The metal foil is discontinuous in a longitudinal direction, but continuous in a torsional direction, thus producing a spiral. The metal foil is wrapped at or near the extreme diameter of the shaft and stiffens the shaft torsionally, thereby increasing the frequency and excitation energies of the torsional modes. The first combined mode is usually the first torsional mode, which often falls into the frequencies deemed by golfers to be harsh.
This aspect improves the bending stiffness of the club while adding mass, such that the first and second frequencies of the shaft and the club remain in the desired range of modal frequency, typically 2-10 hertz. The mass of the foil along the outer portion of the shaft also helps to dampen the torsional impulse of an off-center ball impact, although the club head is often the primary component that dampens this impulse. If designed to do so, the grip can attenuate vibration at higher frequencies.